1) Field of the Invention
This invention relates generally to microelectronic masks and mask fabrication methods and more particularly to phase shift masks and fabrication methods thereof.
2) Description of the Prior Art
As integrated circuit devices such as semiconductor devices become more densely integrated, it may become increasingly difficult to form uniform patterns therein. This problem can be reduced or eliminated by using phase shift mask (PSM) structures rather than conventional photo mask structures. In particular, a Levenson PSM (or alternating PSM) may be used to form a uniform pattern. A Levenson PSM includes an etched portion in a PSM substrate. A Levenson PSM is described in detail in U.S. Pat. No. 5,358,827.
A conventional Levenson PSM is formed by sequentially forming a phase shift layer pattern and a chrome layer pattern which expose spaced apart regions of a PSM substrate. A trench having a predetermined depth is formed in one of the exposed regions. The phase of radiation such as light, which is incident on the region where the trench is formed is shifted by an angle of 180 degree. Thus, the region in which the trench is formed becomes a phase shift region. The phase of a light incident on the other exposed region is not shifted. Thus, the region in which a trench is not formed becomes an unshifted phase region.
Unfortunately, when the light passing through the phase shift region is out of focus, the image of a pattern may deteriorate. Thus, the difference D of a critical dimension (hereinafter, referred to as a CD) between patterns respectively formed by light passing through the phase shift region and the unshifted phase region may become large. In order to solve this problem, an undercut can be formed on the PSM substrate on which the trench is formed, or a material having a high refractivity can be used to reduce the step difference of the trench. However, these methods may not completely solve this problem.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering U.S. Pat. No. 6,410,191B(Nistler et al.) that shows a single trench alternating PSM.
U.S. Pat. No. 5,766,829(Cathey, Jr. et al.) shows a chromeless phase shift mask comprised of a pattern of parallel spaced phase shifters.
U.S. Pat. No. 6,458,495B1(Tsai, et al.) shows a dual trench with undercut, alt-PSM.
U.S. Pat. No. 6,355,399b1(Sajan et al.) shows a method for a dual damascene pattern comprising: exposing a one photoresist layers using a grey tone mask.
U.S. Pat. No. 6,482,554(Matsunuma) shows a method for a dual damascene pattern comprising: exposing two photoresist layers using a grey (tri-tone) mask.
S. Vaidya, Phase-Shifting Photomasks, Semiconductor fabtech, Edition 1, Issued September 1994, S. Vaidya, AT&T Bell Laboratories, Murray Hill, N.J., USA, Website:
http://www.semiconductorfabtech.com/features/lithography/articles/body1.171.php3, May 7, 2003
John S. Petersen, et al., Development of a Sub-100 nm Integrated Imaging System Using Chromeless Phase-Shifting Imaging with Very High NA KrF Exposure and Off-axis Illumination, found on website;
http://www.advlitho.com/content/Papers/SPIE microlith 02/4691-50 Petersen Conley et al.pdf, May 8, 2003, discusses Chromeless Phase shift mask techniques.
Gerold, et al., Multiple Pitch Transmission and Phase Analysis of Six Types of Strong Phase-Shifting Masks, This material was presented at SPIE's 26th Annual International Symposium on Microlithography as presentation number 4346-72, found on website: http://www.advlitho.com/content/Papers/4346-72paper.pdf May 8, 2003. This reference discusses alternating phase shift masks.
Armen Kroyan and Hua-yu Liu, Effects of altPSM Design on Image Imbalance for 65 nm, Semiconductor International, Feb. 1, 2003 http://www.e-insite.net/semiconductor/index.asp?layout=article&articleId=CA273367&spacedesc=webe x)